Thirteen Israeli youths on a bicycle outing were injured today, two of them seriously, when a hand grenade was thrown at them from a rooftop in the West Bank city of Jericho, Israeli military authorities said.

The attack, which the Army command blamed on Arab terrorists, was the most serious to have occurred in several years in Jericho, one of the most popular tourist sites in the occupied territories for Israeli and foreign visitors.

The Army command said the victims, mostly teen-agers from a nature conservation school near the northern Israeli town of Carmiel, had just returned their bicycles to a rental shop in the center of the exclusively Arab city and were gathered near their bus when the grenade landed in their midst. An Arab passer-by also was wounded, officials said.

The Democratic Front for the Liberation of Palestine issued a statement in Damascus saying it was responsible for the attack. An Army spokesman said the explosion was caused by a Soviet-manufactured F1 hand grenade.

Army Maj. Gen. Ehud Barak, chief of the Central Command, said the attack was unusual for Jericho, which has the lowest incidence of terrorism of major West Bank cities.

He speculated that the grenade could have been thrown by the same group that threw a grenade at an Israeli Army bus on Feb. 2, resulting in no injuries.